Heretofore, plain bearings are have been used to support journalled portions and frictional portions of internal combustion engines and compressors of vehicles, ships, etc. By the term "plain bearing" is meant a frictional bearing member such as a journal bearing or thrust washer, and the bearing material used for the plain bearing is required to have, an embedding property, or embeddability fatigue strength, a load resistant property, a friction-resistant property, and a wear-resistant property. In the usual bearing material, a bearing layer constituting a bearing surface, which supports a journal or the like, is formed on a backing material consisting of a strip of soft steel or like material, and one or more intermediate layers are provided between the bearing layer as a surface layer and backing material. The intermediate layers are formed either by a casting process or a sintering process, and they are classified in view of the material into those consisting of copper or copper alloys (hereinafter referred to as the Cu system), and those consisting of aluminum or aluminum alloys (hereinafter referred to as Al system). Bearing materials which include an Al system bearing layer (hereinafter referred to as Al system bearing materials) are lighter in weight than, and economically very superior to, those with Cu system bearing and/or intermediate layers (hereinafter referred to as Cu system bearing materials), so that they are used extensively for journal bearings of automotive internal combustion engines. Recently, however, there have been demands for size reduction and an output increase of automotive internal combustion engines, and particularly, exhaust gas purifiers have been provided to cope with pollution problems. Therefore, the bearing materials more frequently are used under higher loads at higher frictional speed, and at higher temperature. For this reason, Cu system bearing materials are used again in lieu of Al system bearing materials.
The Cu system bearing material, produced either by casting or by sintering, is based on what is commonly termed bronze alloy, which contains Sn incorporated into Cu as the principal component. The matrix of the bronze alloy is superior in the load-resistant property and the wear-resistant property, and these superior properties are utilized for the bearing material.
More particularly, the prior art Cu system bearing material based on bronze alloy contains up to about 30% of Sn. If its Sn content is up to about 14%, it is in a range of 8-solid solution, and the matrices are generally in the form of .alpha.+.delta. crystals. Due to this structure, the bronze alloy is superior in the load-resistant property and the wear-resistant property.
In addition to these properties, however, the bearing material should have other improved properties such as a lubrication property. For this reason, Pb is added in a range of about 20% to 30% in addition to Sn. However, Pb and Cu can form a solid solution in only a very slight range, and seldom will form any solid solution. Therefore, the added Pb is precipitated in the form of spheres or masses in the bronze alloy matrix. Besides, the masses of Pb are united with one another to form continuous bodies. When external forces or loads act repeatedly on the Cu system bearing material as bearing, the material is liable to be broken along continuous bodies. In addition, it is liable to be corroded by the lubricant. Thus, the fatigue-resistant property and the corrosion-resistant property are weakened considerably.
There have been proposed Sn- and Pb-containing Cu system bearing materials which are improved to overcome the drawbacks due to the addition of Pb.
For example, U.S. Pat. No. 3,180,008 discloses a bearing material, in which a multi-layer structure of copper alloy consisting of intermediate and bearing layers is formed on a backing material of soft steel. The multi-layer structure is formed by casting. The surface layer of the multi-layer structure serving as a bearing layer is a lead alloy layer containing, 2% to 10% of In and selectively containing 0.1% to 3% of Cu, 0.001% to 0.25% of Te, 0.5% or less of Ag and/or 0.5% or less of Sb, and the remainder being Pb, while the intermediate layer of the multi-layer structure is a copper alloy layer containg 5% to 35% of Pb and 20% or less of Sn, and the remainder being Cu. In this bearing material, In of the surface bearing layer partly enters the intermediate layer to be coupled to Cu so as to form a Cu-In alloy, thus improving the anti-abrasion while it also is coupled partly to Pb to improve the frictional property. Further, at the time of the disordering operation, when the intermediate layer is exposed partly or totally due to breakage of the bearing layer, the intermediate layer exhibits a bearing function for its matrices having a bronze alloy structure and contain precipitated Pb as the lubricant component. However, the multi-layer structure consisting of the bearing and intermediate layers is fabricated by a casting process. More particularly, the backing material is bent so as to have its opposite edges folded in the shape of a canal, then heating the backing material in, for instance, a reducing atmosphere at about 1,100.degree. C., then forming a casting of melted copper alloy as the intermediate layer on the backing material, and then quenching the system. This is followed by forming a casting of melted lead alloy as the surface layer subsequent to similar heating, and then quenching the system, followed by cutting apart the opposite edges of the backing material. This process of fabrication requires processing of the backing material and repeated casting and quenching, so that it is greatly cumbersome, and the yield is reduced greatly. Besides, the bearing layer of the half bearing, which is formed by working the bearing material, contains Pb, which contains a comparatively large amount of expensive In, so that this bearing material is economically inferior.
U.S. Pat. No. 4,406,857, British Patent No. 658,335 and Japanese Patent Disclosure No. 94,501/1982 disclose Cu system bearing materials, in which bearing and intermediate layers are formed by sintering.
In the bearing material disclosed in the U.S. Pat. No. 4,406,857, a sintered alloy layer containing 8% to 27% of Pb, 0.5% to 10% of Sn, 2% to 10% of Ni, and the remainder being Cu, is formed as a bearing layer or intermediate layer on a backing material consisting of a steel sheet strip.
The matrix of the sintered alloy layer of this bearing material has a bronze structure, in which Cu and Sn form a solid solution, and Pb is dispersed in the matrix. However, the structure is not a cast structure but a sintered structure. Therefore, the adhesion between the alloy layer and backing material is inferior, and Pb is liable to form continuous bodies. By adding Ni, however, the Pb continuous bodies are broken, while Ni forms a solid solution in the matrix, thus improving the mechanical strength. Nevertheless, this bearing material is very expensive for it contains 2% to 10% of expensive Ni. In addition, although Pb continuous bodies are broken, the fatigue strength is not improved much.
In the Cu system bearing material disclosed in British Patent No. 658,335, a Cu alloy layer is formed as a sintered layer on a backing material. In the sintered alloy layer of this material, a portion close to the backing material has as high a content as 4% to 11% of Sn, while the other portion of the bearing surface has as low a content as 1.5% or less of Sn. With this bearing material, Sn is dispersed in the sintered alloy layer portion close to the backing material to increase the adhesion between the alloy layer and the backing material. However, it is cumbersome to vary the Sn content in the thickness direction of the sintered alloy layer. In addition, Pb which does not form any solid solution with Cu, forms continuous bodies, so that the fatigue strength is inferior.
The Cu bearing material disclosed in Japanese Patent disclosure No. 94,501-1982 contains a sintered alloy layer containing 1% to 5% or less by weight of Ni and 0.5% to 3% or less by weight of Sb as well as 8% to 20% or less by weight of Pb and 4% to 10% by weight of Sn, with the remainder being Cu. In this bearing material, Sb is incorporated, with Ni in order to break continuous bodies of Pb which does not form any solid solution with Cu, so as to improve the bearing performance. However, Ni and Sb additions are very expensive, and deteriorate the embedding property or embeddability and these elements have to be added in comparatively large amounts if satisfactory yield is to be obtained. Therefore, this bearing material is inferior in economy.